This invention relates to a belt drive arrangement for advancing a sheet or web of material through the printing station in a thermal transfer printer.
FIG. 1 shows a schematic side view of a conventional thermal printer as disclosed in Japanese koho No. 83/142887, wherein a platen drum 30 covered with rubber or the like is provided with a transverse clamp 7 for engaging the leading edge of a sheet or web 6 of paper or other material to be printed upon, the length of such sheet being less than the outer circumference of the drum. Reference numeral 1 designates a thermal printing head reciprocable to and away from the platen drum by a drive mechanism, not shown, and 8 is an ink ribbon which, in the case of a color printer, would carry successive ink portions of yellow, magenta, cyan and possibly black as disclosed, for example, in U.S. Pat. No. 4,558,329.
In operation, referring to a color printer, the leading edge of the sheet 6 is firmly gripped by the clamp 7 and advanced by the rotation of the drum in the direction of arrow A to a point just past the printing station, whereafter the thermal head 1 is urged against the platen drum, the ribbon 8 is advanced, and the drum rotation is continued to implement the printing of a first color separation on the sheet 6 under the control of selective signals individually applied to the heating elements of the head from an external control source, not shown. Upon the completion of the first color separation printing the head 1 is displaced away from the drum, the ribbon 8 is advanced to present a new color portion at the printing station, the drum with the clamped sheet 6 wrapped therearound is rotated in a reverse direction to return the leading edge of the sheet to its initial or start position, and the previously described operation is repeated to successively print the different color separations on the sheet in registration with each other.
A conventional thermal printer of this type requires a platen drum diameter of at least 160 mm when Japanese Industrial Standard A3 size paper is used, which unduly increases the overall size of the entire device. Furthermore, the large drum diameter attendantly increases the radius of curvature of the drum at the printing station, whereupon some of the integrated circuit drivers and other electronic components mounted on a ceramic substrate of the thermal head are prone to contact the outer surface of the drum. This can be avoided by increasing the size of the ceramic substrate, but at the expense of increased cost for the thermal head assembly.